fanatic

Concise Oxford English Dictionary © 2008 Oxford University Press:
fanatic /fəˈnatɪk/

▶noun a person filled with excessive zeal, especially for an extreme religious or political cause.

■ informal a person with an obsessive enthusiasm for a pastime or hobby.

▶adjective filled with or expressing excessive zeal.

– derivatives
fanaticism noun,
fanaticize (or fanaticise) verb.

word history: The word fanatic derives from Latin fanaticus, meaning ‘of a temple’ or ‘inspired by a god’, from fanum ‘temple’. It was first used in the 16th century, as an adjective describing behaviour or speech that might result from possession by a god or demon; hence the earliest sense of the noun ‘a religious maniac’. Another English word deriving ultimately from fanum is profane, which comes from Latin profanus, meaning ‘outside the temple’.